The present invention relates to an adhesive composition as disclosed in the preamble of the first claim.
Laminated corrugated board is a multilayer material in which one or both of the top and bottom side of a corrugated cardboard core are adhered to a sheet of flat board or paper. In the manufacturing of corrugated board, often use is made of a starch based adhesive. An adhesive layer is applied to the top of the corrugations, following which the tops of the corrugations are adhered to the paper sheet. Thereby it is important that, depending on whether single faced or double backed cardboard is produced, (i) good adhesion is obtained under application of high, respectively relatively low pressure; (ii) extensive contact, respectively restricted contact is established between the surfaces to be bonded. It is furthermore important that when applying the adhesive, its viscosity is sufficiently low, but that after application the viscosity rapidly increases due to gelatinisation of the starch upon heating.
In this respect, EP-A-1.101.809 discloses that gelatinisation may be promoted through addition of sodium hydroxide and a boron compound, for example boric acid or borax. This is done to solve the problem of providing an adhesive composition with a sufficiently high solids content and a reduced water content, with the aim of minimising deformation of the corrugations and energy consumption for evaporating the water from the card board after the adhesive has been applied. It is disclosed that with starch as the binding agent, the upper limit of the dry solids content is determined by the starch concentration as otherwise insufficient water is present to guarantee good adhesion and the viscosity of the adhesive composition becomes too high. The addition of a water insoluble salt in particular calcium carbonate allows increasing the dry solids content to over 46 wt. %. The calcium carbonate is added in a ratio starch:calclum carbonate which is at least between 50:50, preferably between 90:10.
The adhesive composition disclosed in EP-A-1.101.809 however presents the disadvantage that the addition of the filler is expensive and involves clogging of the processing equipment.
FR-A-2784992 discloses an amylaceous adhesive composition for manufacturing corrugated paper board, which contains water, starch and an amount of two groups of active agents. The active agents serve the purpose of increasing the viscosity of the adhesive composition and of improving the wettability properties. The first active agent is a biodegradable soap, the second active agent is a sequestering agent for complexing alkaline earth metal ions such as calcium, magnesium, iron and others. Suitable sequestering agents include polycarboxylic acids and/or phosphonic acid and/or polyacrylc acid, and or nitrilotriacetic acid or their alkaline salts and/or alkaline phosphates and mixtures thereof. The sequestering agent has the function of solubilising any fatty acids remaining in the soap.
Paper board produced using the adhesive composition disclosed in FR-A-2784992 however presents the disadvantage that it shows a high risk to deformation and weakening of the paper board structure, due to increased penetration of the adhesive into the paper. Water has been found to cause deformation of both the corrugated paper and the flat liners.